The present invention relates to an anti-skid control system used on automobiles and more particularly to a safety device for the anti-skid control system suitable for improving the car stability during braking.
Conventional anti-skid control systems have a valve mechanism that can reduce or hold the braking oil pressure while the car is running on roads having low surface friction coefficients, such as icy or snowy roads. With the valve mechanism, the conventional anti-skid control systems can prevent the wheels from getting locked and secure a steering stability without elongating the braking distance.
It is noted, however, that the conventional anti-skid control systems cannot secure the steering stability and the sufficient braking force on all kinds of road surfaces. Particularly when there is a difference in the frictional coefficient between the road surfaces on which the left and right wheels are running, that is, when the car is traveling on the split .mu. road, the control that makes the slip rates of the left and right wheels equal will produce a difference between the braking forces acting on the left and right wheels. This imbalance of braking forces will cause a yaw moment, making the car unstable.
To improve the stability on the above-mentioned split .mu. road, the conventional anti-skid control system performs a so-called select-low control, which makes other braking oil pressures equal to that of the wheel that easily locks. This control, however, produces too small a braking force, increasing the braking distance.
The Japanese Patent Laid-Open No. 1-208256(1989) carries this method a step further and discloses a control which, when the car is unstable with a large yaw rate (yaw angle) increases the cornering force (side force) by reducing the target slip rate of the rear wheels, thereby improving the driving stability of the car.
General anti-skid control systems have a safety device for performing a failsafe processing. One such example is disclosed in the Japanese Patent Laid-Open No. 60-45457(1985). This safety device, when it detects a broken wire of a wheel speed sensor, cuts off the relay of the anti-skid control system to disable a control valve and therefore the anti-skid control. Another example is cited in the Japanese Patent Laid-Open No. 63-46961(1988) and 1-195168(1989). When it detects failure of ground sensor or acceleration sensor, the safety device determines the estimated car speed from a signal of other sensor, i.e., wheel speed sensor, and performs the anti-skid control based on this estimated car speed.
As mentioned above, the anti-skid control system of the Japanese Patent Laid-Open No. 1-208256(1989) improves the cornering force by reducing the target slip rate of the rear wheels when the car is running in an unstable condition with a large yaw rate. However, no consideration is given to ensure a failsafe of the system in the event of a failure of the sensor that detects the yaw rate. Hence, when the sensor for sensing the yaw rate fails and produces an erroneous output, the system controls the car at a reduced target slip rate even when the car is not in the unstable condition, increasing the braking distance.
The safety device for the general anti-skid control system, as disclosed in the Japanese Patent Laid-Open No. 60-45457(1985), disables the control valve upon detection of a sensor failure. This method has a drawback that a yaw sensor failure leads to a system down even when the control valve is normal making the fundamental operation of the anti-skid control system impossible.
Further, the method described in the Japanese Patent Laid-Open No. 63-46961(1988) and 1-195168(1989) uses as a means to detect the car speed the ground sensor or acceleration sensor that has higher precision than the wheel speed sensor. When a sensor failure is detected, this method switches over to the wheel speed sensor that has lower precision. This method therefore cannot be applied where a spare or standby sensor is not used.